This invention relates to reduction of jamming or other interference affecting signal reception and, more particularly, to use of auxiliary beam patterns of an array antenna to provide interference reduction by adaptive main beam nulling, with application to sum and difference pattern processing and to IFF applications.
In operation of radar and identification friend-or-foe (IFF) systems, reception of desired signals may be degraded or obscured by the presence of a variety of forms of jamming signals or other types of intentional and unintentional interfering signals. Many techniques have previously been described for enabling or improving reception of desired signals in the presence of interfering signals, particularly in the context of systems utilizing directional receiving antenna configurations. Such prior techniques typically address sidelobe, rather than main lobe, signal cancellation and may require provision of one or more auxiliary omnidirectional antennas to provide auxiliary signals useful for cancellation processing. Representative prior systems are shown in U.S. Pat. Nos. 3,202,990, 3,881,177, 3,982,245 and 4,044,359.
While early systems may have required adjustments to address particular jamming signals, other systems have employed automated or adaptive nulling techniques to cause a sidelobe null to be effective in the direction from which jamming signals arrive at a receiving antenna. Certain of these systems have employed adaptive feedback loops using complex weighting devices, responsive to least mean square (LMS) processing of an auxiliary signal containing a jamming signal, to develop a weighted signal which, when combined with the received signal, is effective to tend to minimize the presence of the jamming signal in a sidelobe of the received signal. See, for example, U.S. Pat. Nos. 4,280,128 and 4,584,583.
So far as is known, however, there have not previously been provided effective systems for achieving automated cancellation of jamming signals from the main beam of a received signal, and such systems effective in application (a) to systems utilizing sum and difference signal processing, (b) to IFF systems necessitating anti-jam implementation with a minimum of additional circuit elements, (c) to provide simultaneous multi-jammer cancellation, and (d) to combinations of the foregoing.
Objects of the present invention are, therefore, to provide new and improved anti-jam systems and such systems providing one or more of the following capabilities and characteristics:
main beam adaptive nulling, in addition to sidelobe adaptive nulling; PA1 operation without requiring separate auxiliary antennas; PA1 provision of auxiliary beam patterns by use of ancillary signals normally dissipated in main beam formation; PA1 provision of orthogonal auxiliary beam patterns which track main beam steering; PA1 provision of auxiliary signals from beams formed by an auxiliary beam forming network; PA1 simultaneous nulling for both sum and difference beam patterns; PA1 simultaneous adaptive nulling of a plurality of jammers (e.g., four jammers) by parallel LMS control loops; PA1 nulling of higher average power continuous wave or other interference, in contrast to lower average power pulsed IFF or radar signals; PA1 multiple LMS control loop functions implementable by use of time share or multiplexing techniques; and PA1 nulling system implementation suitable for IFF and other applications.